This patent document relates to micro- and nano-scale materials.
Micro/nano-scale propulsion in fluids can be challenging due to the absence of the inertial forces exploited by biological organisms on macroscopic scales. The difficulties are summarized by E. M. Purcell's “scallop theorem”, which states that a reciprocal motion (a deformation with time-reversal symmetry) cannot lead to any net propulsion at low Reynolds numbers. The Reynolds number, Re=ρUL/μ, measures the relative importance of inertial to viscous forces, where ρ and μ are the density and shear viscosity of the fluid, while U and L are the characteristic velocity and length scales of the self-propelling body. Natural microorganisms can inhabit a world where Re˜10−5 (e.g., flagellated bacteria) to 10−2 (e.g., spermatozoa), and they achieve their propulsion by propagating traveling waves along their flagella (or rotating them) to break the time-reversibility requirement, and hence escape the constraints of the scallop theorem. Yet, there is a formidable challenge in engineering nanoscale, complex objects and systems capable of locomotion in fluids, which can be due to the combination of low Reynolds numbers and Brownian motion. Overcoming the challenges and limitations of micro/nano-scale propulsion in fluids can hold important implications.